The presently claimed invention relates to a method of preparing fine grain WC--Co(Ni) powders for cemented carbides.
WC--Co cemented carbides are made by the powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering. The milling operation is an intensive wet milling in mills of different sizes with the aid of milling bodies. The milling time is of the order of several hours up to days. Milling is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed, that the intensive milling increases the reactivity of the mixture which further promotes the formation of a dense structure.
Because of the long milling time, the milling bodies wear and contaminate the milled mixture which, has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling, a nonhomogeneous rather than an ideal homogenous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure, sintering has to be performed at a higher temperature than the theoretical.
An alternative way is to start from an intimate mixture of cobalt and tungsten, which subsequently is carburized. The mixture can be obtained through the formation of a composite metal salt in a chemical process step. U.S. Pat. No. 3,440,035 discloses such a method of preparing cemented carbide powder characterized in that a suspension or solution of ammonium paratungstate (APT) in water is mixed with a nitric or hydrochloric aqueous solution of, e.g., cobalt. The mixture is neutralized with ammonium hydroxide and reacted at a temperature of from 20.degree. to 80.degree. C. The pH after the reaction, should be within the range 4.5-8. The resultant composite salt powder if filtered off and processed into WC--Co cemented carbide.
Our U.S. patent application Ser. No. 08/465,356 filed Jun. 5, 1995 , now U.S. Pat. No. 5,594,929 discloses an easy method of preparing a powder containing WC and cobalt and/or nickel. APT powder and a powder of a basic salt of cobalt and/or nickel are mixed in water. The suspension is stirred to react at temperatures ranging from room temperature to the boiling point of the solution whereby a precipitate is formed, which precipitate is filtered off, dried and finally reduced to a metallic powder.
According to copending U.S. patent application Ser. No. 08/464,564 filed Jun. 5, 1995 (Attorney Docket No.: 024444-132), a powder containing W and Co and/or Ni can be prepared by reacting APT and a soluble salt of Co(Ni) in water at temperatures from room temperature to the boiling point of the solution keeping the solution pH at a constant level during the chemical reactions. The powder is filtered off, dried and reduced to a metallic powder which may be further carburized to form a powder containing WC and Co and/or Ni.
In our U.S. patent application Ser. No. 08/464,965, now U.S. Pat. No. 5,584,907, a method is disclosed for preparing a powder containing W and Co and/or Ni from APT and a soluble salt of Co(Ni) by a chemical reaction in a water suspension at temperatures from room temperature to the boiling point of the solution, whereafter, the formed powder is filtered off, dried and reduced to a metallic powder. The method is characterized in that additional metals from groups IVa, Va or VIa of the periodical table of the elements are added to the suspension as compounds like oxides, hydroxides, soluble or insoluble salts.